This invention relates generally to motion compensation, and more particularly to improvements in heavy duty compensating devices making them simpler, more effective and reliable. More specifically it concerns multiple actuators and sheave support and control mechanism.
There is need for simple, effective reliable, heavy duty, motion and load compensating equipment. For example, helicopter landing pads should support a predetermined load and dissipate additional loading, to compensate for and nullify additional forces exerted as a result of deck "heave", on a vessel. A desirable "shock deck" should also compensate for a "hot" landing or inadvertent rapid descent rate, of the helicopter, and which might otherwise adversely affect the structural integrity of the deck support structure.
In the case of a floating offshore drilling vessel, it cannot inherently provide a constantly stable platform as related to the sub-sea well head. In this regard, a stable reference is required for landing and retrieving of wellhead and blow out prevention equipment, control of string weight on the drill bit in the hole, landing of casing and liner, coring, well logging and fishing. There is need for nullification of the effects of rig/platform heave in response to swelling seas, and for compensating apparatus that will maintain a predetermined lifting force.
Prior Drill String Compensators (D.S.C's) sometimes called heave compensators, are of two types:
1. Block mounted, or PA0 2. Crown mounted PA0 1. Stroke/compensation length is equal to rod length or must incorporate chains and sheaves which add additional wear/failure areas. PA0 2. Rig heave compensation causes compression or expansion of compressed air, which in turn causes an inverse reaction in the compensating force applied.
Block mounted compensators, substantially increase the weight applied to the draw works, require precise alignment of derrick track and dollys, and represent a substantial change in the deck loading arm by their movement up and down the derrick. Crown mounted compensators, overcome these major disadvantages, but still add a significant weight to the crown of the derrick. These two methods share some common disadvantages: